This application claims the priority of German Patent Application, Serial No. 101 18486.7 filed Apr. 12, 2001, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to an injection unit, and more particularly, to an injection unit for an injection molding machine with a continuously operating plasticizing unit, a piston-type injection unit, and means for intermediate storage of the melt. The invention has applications for forming compounds of plastic melts and additives, in particular to introducing natural or long fibers, as well as for injecting the compound into a closing unit of an injection molding machine.
German Pat. No. DE 38 41 728 C1 and Swiss Pat. No. CH 423 197 describe the introduction of fillers and additives into a polymer matrix using single screw or multiple screw extruders. According to Swiss Pat. No. CH 423 197, long fibers (rovings) are introduced into the base material by feeding the rovings with a rotating screw. The rotation speed of the screw and the quantity of rovings determine the fiber fraction in the plastic melt. Kneading elements can be used to better homogenize the fibers in the melt and to bring the fibers to a desired length.
Not only continuous fibers can be introduced in the plastic melt, but cut glass fibers or powdered materials, such as chalk, talcum and the like, can also be fed through additional fill ports. The feed characteristic can be improved through the use of screw conveyors (German Pat. No. DE 38 41 728 C1) or feed screws (German Pat. No. DE 36 00 588 C1).
A continuous operating mode of the plasticizing unit is recommended for producing homogeneous plastic melts with single screw or multiple screw machines (extruders or kneaders). However, injection molding is an intermittent process, so that measures have to be taken to mate the continuous operation of the plasticizing unit to the intermittent operation of the injection molding machine.
It is known from German Pat. No. DE 198 59 472 A1 to drive fillers and additives into a polymer matrix using a continuously operating synchronized dual screw extruder and to supply the continuous melt flow alternatingly to two piston-type injection units, wherein each of the piston-type injection units is connected to a corresponding closing unit which houses the injection molds. In the described system, the melt exiting the plasticizing unit enters branch lines. Each branch line leads to a respective one of the two piston-type injection units, wherein the screw accumulator can be alternatingly connected to the two piston-type injection units through a switching valve. In operation, the melt flow, controlled by the switching valve, initially enters the screw accumulator of the first piston-type injection unit. After the screw accumulator is completely filled, the switching valve assumes an intermediate position where it interrupts the connection between the screw accumulator and piston accumulator. By subsequently applying pressure to the working piston of the piston-type injection unit, the melt which is located in the piston accumulator and permeated with long fibers is injected via an injection nozzle and a gate channel into the mold cavity of the injection mold. The pressure profile necessary to produce the injection molded part in the mold cavity is established by controllably applying a hydraulic pressure to the work piston.
U.S. Pat. No. 5,454,995 discloses supplying a plastic melt from a continuously operating extruder alternatingly to two piston-type injection units via a switching valve and connecting lines having check valves. A melt outlet line extends from each of the piston accumulators of the two piston-type injection units which hold the melt. The melt outlet lines terminate in a corresponding additional switched check valve, through which the melt enters a connecting channel to be transported to the injection mold. In operation, the two switching valves are controlled so that the injection mold is filled and maintained under pressure while the melt is removed from the accumulator of a first piston-type injection unit and the accumulator of the other piston unit is filled with new melt.
A system with two parallel piston-type injection units, as described in the above-referenced Pat. Nos DE 198 59 472 A1 and U.S. Pat. No. 5,454,995, is relatively complex and difficult to control, because depending on the injection molding cycle each piston-type injection unit has to be filled with at least the melt volume that is to be injected. The plasticizing capacity has to be adjusted to correspond exactly to the melt quantity required for the injection molding cycle. If the cycle in the injection molding process is interrupted and the first injection piston has not yet retracted, then the expelled melt cannot be buffered after the second injection unit is filled, unless the extruder capacity is changed. This causes the pressure in the extruder to increase, requiring an immediate shutdown of the extruder. The melt material remaining in the extruder can thereby be damaged, which tends to be critical when processing natural fiber products. The potential necessary interruption of the production process may reduce the system uptime due to the need for restarting the system and scrapping parts. Moreover, the switching valve located before the piston-type injection units can causes overlapping mass flows and pressure peaks in the plasticizing unit.
German Pat. No. DE 1 142 229 discloses the operation of an injection molding machine with an injection unit which has a continuously operating extruder with a plasticizing unit and an intermittently operating piston-type injection unit, as well as means for intermediate storage of the melt. The melt produced by the continuously operating extruder initially flows through a feed line to a melt reservoir which has a piston connected to a pressure reservoir. The plastic melt flows from the reservoir through a line and a check valve into the piston accumulator of the piston-type injection unit, from which the plastic melt is removed by applying pressure to the work piston and injected through a nozzle into the injection mold. During the injection phase, the check valve closes the melt line between the melt reservoir and the piston-type injection unit, and the melt supplied by the continuously operating extruder displaces the pressure piston in the melt reservoir. As a result, the remaining pressure medium in the pressure reservoir, such as air or gas, is pressurized until the injection piston begins its return motion after the injection phase is completed and the piston accumulator is ready to receive a new melt. The pressurized pressure medium causes the pressure piston in the melt reservoir to transfer the plasticizing plastic from the melt reservoir to the piston accumulator; simultaneously, melt is also transferred from the extruder to the piston accumulator via the reservoir.
German Pat. No. DE 197 15 229 A1 discloses an injection unit of the aforedescribed type, which has a cylindrical melt reservoir with a work piston instead of a pressure reservoir. In one embodiment, the cylindrical melt reservoir is filled with a melt from a lower side near the cylinder base, causing the work piston to be pushed upwardly. In another embodiment, the melt can be introduced from an upper side and a melt distributor can be placed in the cylinder, causing the melt to flow uniformly downward around the piston. For discharging the melt, the work piston moves downward and is held in place in the lower position, while additional melt produced by the extruder of the plasticizing unit flows along the piston directly into the piston accumulator of the piston-type injection unit.
The two injection units described above have the disadvantage if the cycle is interrupted during the injection molding process and the injection piston is not yet retracted, the discharged melt can only be buffered in a quantity equal to the volume of a single melt reservoir. The melt flow can only be controlled by varying the plasticizing capacity of the extruder. Depending on the settings for the pressure reservoir in German Pat. No. DE 1 142 229 or the dimensions of the piston cylinder unit of DE 197 15 229 A1, the pressure in the extruder will sooner or later increase, making it necessary to disconnect the extruder, which has the adverse effects discussed above with reference to German Pat. No. DE 198 59 472 A1 and U.S. Pat. No. 5,454,995.
It would therefore be desirable and advantageous to provide an improved injection unit for injection molding machines with a continuously operating plasticizing unit and piston-type injection unit, which obviates prior art shortcomings and is capable to react more reliably and more flexibly to interruptions of the production process while requiring a complete shut down only in the event of a major malfunction.
According to one aspect of the present invention, an injection assembly for an injection molding machine with a continuously operating plasticizing unit includes a piston-type injection unit; a plurality of melt reservoirs for storing a melt, with the melt reservoirs disposed between the plasticizing unit and the piston-type injection unit and having melt inlet openings; and melt lines merging with the melt inlet openings and connecting the plurality of melt reservoirs with one another and with the plasticizing unit, with each melt reservoir having an actuatable work piston for discharging the melt, wherein the work pistons can be moved into a closure position to close the melt inlet openings in the melt reservoirs for the purpose of diverting a flow of the melt.
According to another aspect of the invention, an injection assembly for an injection molding machine with a continuously operating plasticizing unit includes a piston-type injection unit receiving melt from the plasticizing unit and having a cylinder, which defines an interior space, and an injection piston moveably received in the cylinder and dividing the interior space in an anterior chamber, representing an injection space, and a posterior chamber, representing a melt reservoir, wherein the cylinder includes a melt inlet opening so located at a side of the cylinder that the anterior chamber is filled with melt, when the injection piston is disposed behind the inlet opening, and that the posterior chamber is filled with melt, when the injection piston is located in front of the inlet opening, wherein the injection piston has at least one overflow valve constructed to open during a return motion of the piston so as to allow a flow of melt from the melt reservoir into the injection space, and to close during a forward motion of the injection piston.
According to yet another aspect of the invention, an injection unit for an injection molding machine with a continuously operating plasticizing unit includes a piston-type injection unit receiving melt from the plasticizing unit and having a cylinder, which defines an interior space, and an injection piston moveably received in the cylinder and dividing the interior space in an anterior chamber, representing an injection space, and a posterior chamber, representing a melt reservoir, said cylinder including a melt inlet opening so located at a side of the cylinder that the anterior chamber is filled with melt, when the injection piston is disposed behind the inlet opening, and that the posterior chamber is filled with melt, when the injection piston is located in front of the inlet opening; and passageway means including at least one overflow channel to connect the melt reservoir with the injection space so as to allow a flow of melt from the melt reservoir into the injection space during a return motion of the injection piston.
Providing several melt reservoirs advantageously increases the total volume Vtot of bufferable melt. The melt reservoirs are also provided with actively actuatable work pistons, so that the reservoir volumes (Vs) and the outflow (discharge) velocity (dVs/dT) of the individual melt reservoirs can be adjusted individually and separately from each other. This makes the process highly flexible in the event of a malfunction. Moreover, the reservoir volumes Vs and the outflow velocity dVs/dT of the individual reservoirs can be optimally matched to the injection molding process, for example adapted to the size of the injection molded parts, the cool-down time, the removal time for the formed parts and other characteristic parameters. Switching valves are eliminated since the melt flow is switched directly by the work piston. This arrangement reduces the technical complexity of the apparatus and the control system. In addition, long fibers or natural fibers which can be damaged when passing through the valves are also processed more advantageously.
Advantageously, the need for separate melt reservoirs can be obviated by integrating the melt reservoirs in the piston-type injection unit and forming the space behind the piston as a melt reservoir with sufficient capacity. The melt reservoir typically holds approximately half of the total volume of the cylinder. When the piston has reached the forward end position in the filling phase, essentially the entire cylinder volume is available as melt reservoir, except for the piston elements. This corresponds to approximately twice the shot weight of the injection unit.
According to another feature of the present invention, overflow valves can be incorporated in the injection piston of the piston-type injection unit.
According to yet another feature of the present invention, overflow channels may be provided in the cylinder of the piston-type injection unit so as to allow the plastic melt to flow from the space behind the piston (melt reservoir) to the piston accumulator (injection space).